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When I start up my Z3 when it has become cold (such as overnight) it makes a squeek sound like a belt that has to warm up and then stop making noise. It lasts about three minutes.
My mechanic has noted that it is time for a new timing chain (or belt???) and a radiator flush. Could this be the cause?
It is not a crazy, insanely loud sound, just sort of high pitched whirring.
Thanks!

Belts can squeak. Get a tube of belt dressing and dab some on the belts - it will spread itself around. I don't like the spray dressings because only a little gets on the belt.
If the belts are still 1996, they should be changed.
The timing chain is permanent, internal, oil-lubricated and does not squeak.
Flushing the radiator will have no effect on the noise.

Additionally, if you are having belt issues, it could be an indication that they are original and need to be replaced. That's also an indication that the cooling system has not been replaced. After about 80,000 miles, the plastic pieces in these cooling systems becomes brittle, can break catastrophically, and quickly ruin an engine.
It may be time for a complete cooling system replacement including: belts, hoses, water pump, radiator, coolant reservoir, thermostat, thermostat housing, and if it's a 1.9: two more plastic pieces on the engine.
If that has been done, I would consider idler pulleys. A mechanic can usually quickly isolate the noise using a piece of hose or stethoscope.

Standard wisdom says not to spray belt dressing on serpentine belts (check the warning on the can). You might try spraying pain old water on the belt to see if this quiets it down. If it does, then you need to replace your belt(s). I had some temporary relief on a squeaking belt on a non-BMW car by putting some Comet cleaner on the belt. It removed the glaze from the belt...but in end, the belt needed to be changed.

Standard wisdom says not to spray belt dressing on serpentine belts (check the warning on the can)....you might try spraying pain old water on the belt to see if this quiets it down.

I always use the dressing in a tube, put a dab on a finger tip, and rub a little of that here and there on the accessible sections of the belts. That's all it takes. I would not spray belt dressing because most of it gets all over the engine with very little getting on the undersides of the belts. I think water might momentarily quiet a belt. But if the belt is slipping, the lasting adhesion of dressing is needed rather than the temporary lubrication of water.

Dressing is a stop-gap measure to quiet noise from a loose or misaligned or worn belt. The condition causing the noise needs to be addressed sooner or later.